Add basic completion for 'iw'

[Synray]

Only completes up to the 4th level of a command, at the moment. And only for commands starting with iw dev. The rest will come next, though this completion covers all of the most frequently used, as well as the iw help commands.
External: sed and _available_interfaces (iwconfig / ip).

[Synray]

Completion for all "iw dev" commands.

I'll probably try to refactor the duplicate completions.
It's not wrapped at column 79 yet, but besides that is the code style correct?

[scop]

Sorry about the delay. Regarding code style, there are a couple of things to do:

• use spaces (only) for indentation, not tabs
• use space between $( and the command following it
• use == instead of = within [[ ]]

I'll have a proper look later, but in the meantime, fixing the above wouldn't hurt.

Something I will also want for this before inclusion is tests for the test suite, at least a basic one.

[Synray]

Sorry it's been a while since I worked on this. I don't really use iw anymore, but since I think iw's horrendous syntax (and tendency to barf unsorted help to stdout when it fails) creates a drastic need for bash completion, I'll try to finish this.

I made the style changes you requested, apart from wrapping all the lines to 79 characters (which I'll do once the code is relatively stable). I also added completion for iw phy. The 3 commits were then squashed into one.

I'll add a test case next.

[Synray]

Added a test, I looked at some other tests, but the length of each varies a lot, so let me know if I should add more to it.